Field of the Invention
The present invention relates to a magnetic-disk glass substrate and a magnetic disk.
Background Art
Nowadays, personal computers, digital Versatile disc (DVD) recorders, and the like have a built-in hard disk drive (HDD) for data recording. In particular, a magnetic disk in which a magnetic layer is provided on a glass substrate is used in a hard disk drive that is used in a device premised on portability, such as a notebook-type personal computer, and magnetic recording information is recorded on or read from the magnetic layer with a magnetic head that flies slightly above the surface of the magnetic disk. A glass substrate is unlikely to be plastically deformed compared with a metal substrate (aluminum substrate) and the like, and thus is preferably used as the substrate of magnetic disks.
Moreover, the density of magnetic recording has been increased to meet the demand for an increase in the storage capacity of hard disk drives. For example, the magnetic recording information area has been made smaller using a perpendicular magnetic recording system that causes the direction of magnetization in the magnetic layer to be perpendicular to the surface of the substrate. This makes it possible to increase the storage capacity per disk substrate. In such a disk substrate, it is preferable that the substrate surface is made as flat as possible and the direction in which magnetic particles grow is arranged in the vertical direction such that the direction of magnetization in the magnetic layer faces in a substantially perpendicular direction relative to the substrate surface.
Also, in order to further increase the storage capacity, by using a magnetic head equipped with a dynamic flying height (DFH) mechanism to make the flying height of the magnetic head from the magnetic recording surface extremely short, the magnetic spacing between the recording and reproducing element of the magnetic head and the magnetic recording layer of the magnetic disk is reduced, thus further improving the accuracy of the recording and reproducing of information (improve the S/N ratio). Also in this case, it is required to make the surface unevenness of a magnetic-disk substrate as small as possible in order for the magnetic head to stably read/write magnetic recording information over a long period of time.
Servo information to be used to locate the magnetic head at a data track is recorded on the magnetic disk. It is conventionally known that when the roundness of an edge surface of the magnetic disk on the outer circumferential side (also referred to as “outer circumferential edge surface” hereinafter) is reduced, the magnetic head flies stably, the servo information is favorably read, and thus the magnetic head stably reads/writes information. For example, the technique described in JP 2008-217918A discloses a magnetic-disk glass substrate in which the roundness of the outer circumferential edge surface is 4 μm or less. With this glass substrate, the durability against load/unload (LUL) testing is improved by reducing the roundness of the outer circumferential edge surface.